Determining the source of spatially variable water chemistry in perennial tributaries in the Grand Canyon, Arizona, USA: influences from water-rock interaction and marine evaporite dissolution

Abstract

Anthropogenic climate change poses an imminent threat to limited freshwater resources in the southwest United States as surface temperatures are expected to rise and precipitation events become less predictable. This in turn will increase the reliance on local freshwater resources, and, if managed carelessly, may threaten sensitive aquatic and riparian ecosystems where groundwaters emanate at springs that supply perennial tributaries. Understanding the influences on perennial tributary water chemistry is critical to appropriate management of freshwater resources and riparian ecosystems that sustain native species. Geochemical signatures from water-rock interaction provide a means to determine primary influences on variability of water chemistry in groundwater and surface waters. To characterize the water chemistry variability in perennial tributaries of the Colorado River in the Grand Canyon region, hydrochemical data, geochemical models, and S isotopes provide substantial evidence for water-rock interactions being the primary influence on water chemistry. A hierarchical cluster analysis grouped perennial tributaries based on major ion chemistry, yet other characteristics not included as variables, such as TDS, geospatial extent, and sulfur isotopes (del34S) generally follow trends defined by the clustering. Piper plots coupled with geochemical models supports the hypothesis of water rock interactions controlling water chemistry variability. The twenty sampled waters range in composition from Ca/Mg-HCO3, to Ca-SO4, to Na-Cl type waters suggesting varying degrees of influence from carbonate (dolomite and limestone) and marine evaporite (gypsum and halite) dissolution on water chemistry. Geochemical models were created to characterize evolution of groundwater through carbonate aquifers with or without evaporites present, which matches observed chemical composition variability. Sulfur isotopes also support a model of variability in water chemistry induced by water-rock interaction without the need for an additional source of sulfur from mantle derived fluids (H2S(gas)) as has been hypothesized in previous studies in this region. Results from sulfur isotopes determined 7 of the twenty perennial tributaries analyzed for del34S have dissolved sulfate derived from overlying marine evaporite rocks of Permian age. These tributaries generally represent high TDS waters (550 to 1,600 mg/L and one tributary with 200 mg/L) indicating water-rock interaction with marine evaporites is the primary influence on TDS. Three tributaries are only influenced by the dissolution of carbonates, while two tributaries are influenced by likely sulfide mineral oxidation, though influence of H2S deep magmatic/mantle gases that also source CO2 to the waters has been proposed in previous studies and cannot be ruled out without additional chemical and isotopic analysis of gases in spring waters. Two groups from the cluster analysis represent seven tributaries that fall between del34S +6 to +8 per mille and are relatively low TDS waters (200 to 500 mg/L) with small concentrations of dissolved sulfate (12 to 41 mg/L). These groups may be influenced by a mixture of marine evaporites and sulfide minerals (or potential H2S gas). The contributions to dissolved sulfate in these tributaries is explored and modeled, but requires more in depth analysis to fully resolve.